An automotive seat is normally equipped with a seat belt for restraining the vehicle occupant from being thrown forward in case of a vehicle crash or the like. Also, a seat belt is normally equipped with an emergency locking retractor (ELR) device which would not restrain the motion of the vehicle occupant under normal conditions but locks up the winding shaft for the seat belt only in case of a sudden stop or a vehicle crash. The ELR device is capable of locking up the winding shaft in a very short time period upon detecting a deceleration level higher than a prescribed value, and thereby prevents the seat belt from being pulled out any further.
However, even when the winding shaft is locked up, it is still not possible to prevent the seat belt from being pulled out by a certain amount due to the tightening of the part of the seat belt wound around the winding shaft. In view of this fact, various pretensioner devices have been proposed which rapidly moves the buckle connected to the seat belt in the direction to tension the seat belt, and thereby increases the restraining force of the seat belt even further (see U.S. Pat. No. 4,705,296). A pretensioner device is normally equipped with a reversing preventing device typically consisting of a ratchet device or a one-way clutch, separate from that for the ELR device, to prevent the buckle from reversing its motion.
Such pretensioner devices may use high pressure gas produced from a chemical reaction as a power generator for pulling the buckle, as proposed in U.S. patent application Ser. No. 08/847,272 now U.S. Pat. No. 5,984,357 issued on Nov. 16, 1999, which is assigned to one of the assignees (applicants) of the present application.
A conventional power generator 30 using high pressure gas is illustrated in FIG. 7. A cylinder 31 fixedly secured to a base member of a seat internally defines an inner bore 31a having a reduced diameter portion 31b at an intermediate part thereof, and receives a piston 32 from a base end thereof. A free end 32a of the piston 32 which is integrally attached to the main part, for instance, via a screw thread projects from an open end of the cylinder 31 and engages a buckle via a link member or the like. The base end 32b of the piston 32 is provided with a somewhat larger diameter, and engages the inner circumferential surface of the cylinder 31 via an O-ring 33. The open end of the cylinder 31 is tightly fitted with a cap 34 having a central opening 34a. This opening 34a slidably engages an intermediate part of the piston 32, and provides a bearing surface for the piston rod as the piston 32 moves.
The other end of the inner bore 31a of the cylinder 31 receives a gas generator 35 which includes a propellant and an ignition fuse. The open rear end of the cylinder 31 is crimped over a retaining plate 37 which retains the gas generator 35 and serves also as a seal.
According to this structure, the assembly is completed with the cylinder 31 fully received in the inner bore 31a as illustrated in FIG. 7. However, if the gap L between the base end portion 32b of the piston 32 and a shoulder surface of the reduced diameter portion 31b is too small, it may not be possible properly accommodate the cumulative dimensional errors of the component parts of the pretensioner device (for instance, the connecting portion of the link member, and the cylinder retaining portion with respect to the base member), and the positional errors of the link member. Also, when there is a play in the connecting portion between the free end portion 32a of the piston 32 and the link member, and the connecting portion between the link member and the buckle, successive collisions may occur in the connecting portions upon activation of the gas generator 35 due to the presence of a play in each connecting portion, and the resulting impacts may produce localized plastic deformations in various parts. Such deformation could lead to the loss of the energy of the high pressure gas. Therefore, the gas generator 35 is required to be larger than desired for ensuring a sufficient drive force for the buckle, and the power generator 30 tends to be larger than desired.
According to this structure, the power generator 30 is transported and stored typically with the piston 32, the cylinder 31 and the gas generator 35 assembled together. If the gas generator 35 is inadvertently activated in such a state, the piston 32 will be projected at high speed, and proper measures are therefore required to be taken for storage and transportation. This means a reduced space efficiency, and an increase in the time required for assembly. Also, because the piston 32 is relatively massive, the cap 34 is required to be strong enough to prevent the piston 32 from being projected from the open end of the cylinder 31 in case of an inadvertent activation of the gas generator 35, and the power generator inevitably becomes bulky and massive.